MAVO Year 4 · Gas exchange and excretion · Section 14.2

The skin and subcutaneous tissue

Structure and functions of skin

The skin protects against physical damage, excessive water loss and many pathogens. It also provides partial protection against ultraviolet radiation and contributes to temperature regulation and sensation. Its two main layers are the epidermis and dermis, above the subcutaneous tissue.

Living epidermis and the horny layer

Cells in the lower epidermis divide to renew the surface. As cells move outwards, they fill with keratin and eventually form the horny layer, consisting mainly of dead, flattened cells. This barrier limits water loss and helps resist damage and infection. Repeated friction can thicken it into a callus, for example on the soles.

Pigment and sunlight

Melanocytes produce melanin, passing it to surrounding cells. Melanin influences skin colour and absorbs some ultraviolet radiation. Sun exposure can increase melanin production and cause tanning, but tanning does not eliminate UV damage or make further exposure harmless.

Hair and sebum

Hairs grow from follicles extending down into the skin. Sebaceous glands, usually associated with follicles, release sebum, an oily secretion lubricating hair and skin and contributing to surface defence. Small arrector pili muscles can pull hairs upright, producing goosebumps.

Dermis and subcutaneous tissue

The dermis contains connective tissue, blood vessels, nerves, receptors, glands and parts of hair follicles. Beneath it, subcutaneous tissue contains variable amounts of fat, providing energy reserves, cushioning and insulation.

Temperature regulation

Core temperature is usually maintained near 37°C, with normal variation. Heat is generated by metabolism and lost through processes at the body surface and breathing.

When warm, increased flow through skin vessels allows greater heat transfer to the surroundings, and sweat evaporation removes heat. When cold, reduced skin blood flow limits heat loss. Shivering increases heat production through muscle activity. Raising hairs traps insulating air effectively in many furry animals, but human goosebumps have little insulating effect.